Spray deposits and losses in different sized apple trees from an axial fan orchard sprayer: 1. Effects of spray liquid flow rate

Abstract

Spray deposit and off-target contamination distribution measurements were made in apple trees of different sizes and row spacings for an axial fan sprayer with different spray liquid flow rates ( c 3.8, 11, 29 l min −1) but otherwise constant conditions (air flow=11.3 m 3 s −1; speed=5.8–5.9 km h −1, VMD=156–159 μm). Leaf deposits (normalised by the applied dose) were not affected by the spray liquid flow rate except on young dwarf trees at the high flow rate due to saturation. The volume retained on the crop increased with tree size. Deposits on individual leaves were approximately log normally distributed. Variability was usually marginally greater for the low flow treatment than for the medium or high flow treatments and usually greater in the larger than in the smaller trees. Mean deposits on the undersides of leaves were about double those on upper surfaces. The mean percentage of the surface area of water sensitive papers in the centre of the tree covered with spray varied in direct proportion to the spray liquid flow rate. Spray cover was least in the larger trees. Deposits on fruitlets were affected significantly by spray liquid flow rate. The high flow rate treatment gave the smallest deposits on fruitlets due to the deposit exceeding local saturation conditions. The deposits on fruitlets tended to be greater in the smaller than in the larger trees. Normalised spray drift deposits on vertical, 10 m tall polythene sampling lines positioned 5 m downwind of the sprayer were somewhat greater for the low flow rate than the medium and high flow rate treatments but differences were at least partly due to differences in wind direction. High values occurred where the wind direction was perpendicular to the tree rows, low values where the wind direction was parallel to the tree rows. Drift generally decreased with increasing tree size. There were no consistent differences in normalised ground deposits due to spray liquid flow rate, though ground deposits were greater in the smaller trees than in the larger trees. As a whole, these results showed that wide variation in spray liquid flow rate had only small effects on normalised spray deposits on the tree, on spray drift and on losses to the ground but greatly affected spray cover which varied in proportion to the spray liquid flow rate. This implies that the spray liquid flow rate for foliar application of an agrochemical should be determined by the degree of spray cover that is needed for adequate efficacy. Furthermore, the pesticide dose rate may be conveniently adjusted in direct proportion to the spray liquid flow rate at constant concentration for deposits lower than the target saturation level.